Okay to be fair I should really have updated this a bit more.
The noise is broadband - it affects multiple bins on the upstream path and affects the downstream slightly too, but to a far lesser extent than the upstream.
Affected downstream bins manage 5+ bits / bin. SNR on them is considerably higher than the SNR on the upstream bins affected.
All this implies to me that the noise / issue is likely sourced from near the exchange, which is far from impossible given the placement of the exchange - an industrial estate.
In any case it's academic. This is in the 'can't fix' section for Openreach and will be resolved once FTTC is in place.
Currently struggling with a 120kbps upstream barely holding on. Not great.